[Windows] wifidirect: Stop checking for Windows.md
[Windows] wifidirect: Stop checking for Windows.md

Synchronously running a command to check if `Windows.winmd` is present
makes it more difficult to port Crosswalk on Windows to GN, as we would
at the very least need to do the same thing through a Python script and
a synchronous script call.

We can safely assume that `Windows.md` is always present though, as it
is installed by the Windows 10 SDK required by Chromium as a build
dependency.

BUG=XWALK-7336

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